This invention relates to active matrix electroluminescent display devices comprising an array of electroluminescent display pixels. More particularly, the invention concerns an active matrix electroluminescent display device comprising an array of display pixels with each pixel comprising an electroluminescent display element and a driving device for controlling the current through the display element based on a drive signal applied to the pixel during an address period and stored as a voltage on a storage capacitance connected to the pixel driving device.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional III-V semiconductor compounds. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of an electroluminescent material, for example a semiconducting conjugated polymer, sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer. The polymer material can be fabricated using a CVD process, or simply by printing or a spin coating technique using a solution of a soluble conjugated polymer.
Organic electroluminescent materials exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore be used in passive type displays.
However, the invention is concerned with active matrix display devices, with each pixel comprising a display element and a switching device for controlling the current through the display elements. Examples of an active matrix electroluminescent display are described in EP-A-0653741 and EP-A-0717446. Unlike active matrix liquid crystal display devices in which the display elements are capacitive and therefore take virtually no current and allow a drive signal voltage to be stored on the capacitance for the whole field period, the electroluminescent display elements need to continuously pass current to generate light. A driving device of a pixel, usually comprising a TFT (thin film transistor), is responsible for controlling the current through the display element. The brightness of the display element is proportional to the current flowing through it. During an address period for a pixel, a drive (data) signal in the form of a voltage or current signal determining the required output from the display element is applied to the pixel and stored as a corresponding voltage level on a storage capacitance which is connected to, and controls the operation of, the current controlling drive device with the voltage stored on the capacitance serving to maintain operation of the switching device in supplying current through the display element during the period, corresponding to a field period, until the pixel is addressed again.
A problem with known organic electroluminescent materials, particularly polymer materials, is that they suffer ageing effects whereby the light output for a given drive current is reduced over a period of time of operation. While in certain applications such ageing effects may not be critical, the consequences in a pixellated display can be serious as any slight variations in light output from pixels can easily be perceived by a viewer.
It is an object of the present invention to provide an active matrix electroluminescent display device in which this problem is overcome at least to an extent.
According to the present invention there is provided an active matrix electroluminescent display device comprising an array of display pixels each comprising an electroluminescent display element and a driving device for controlling the current through the display element based on a drive signal applied to the pixel during an address period and stored as a voltage on a storage capacitance connected to the driving device, which is characterized in that each pixel includes electro-optic adjustment means which is responsive to light produced by the display element during addressing and arranged to adjust in the address period the voltage signal stored on the capacitance in accordance with the light output level of the display element.
Thus, by means of the electro-optic adjustment means, the stored signal voltage for determining the light output level of the display element in the drive (display) period following addressing is adjusted according to the light output characteristic of the display element, providing a feedback variable, whereby the drive level set on the pixel can compensate for the effects of ageing of the display elements so that a desired light output level from a display element for a given applied drive signal is substantially maintained regardless of possible variations in the drive current level/light output level characteristics of individual display elements in the array.
In the case where the drive device of a pixel comprises a TFT (thin film transistor) as is usual in active matrix electroluminescent display devices, the invention offers a further important advantage. The drive current for a display element is determined by the voltage applied to the gate of the TFT, corresponding to the voltage stored in the capacitance. This drive current therefore depends strongly on the characteristics of the TFT and variations in the threshold voltage, mobility and dimensions of the individual TFTs of pixels of the array, for example, due to manufacturing processes, can produce unwanted variations in the display element currents and hence in the light levels produced, leading to non-uniformity in the display output. The effect of the electro-optic adjustment means in controlling the stored voltage signal will also compensate for such variations in TFT characteristics.
Although the invention is particularly beneficial in devices using polymer LED materials, it can of course be applied to advantage in any electroluminescent device in which the electroluminescent material similarly suffers ageing effects resulting in lower light output levels for a given drive current over a period of time of operation.
Preferably the electro-optic means regulates a current flowing in the pixel during the address period according to an applied data signal upon which current the voltage stored on the capacitance following the address period is dependent.
In a preferred embodiment, the electro-optic means comprises a photoelectric device connected to the storage capacitance via a switch device, for example, another TFT, which is arranged to be closed during the address period so as to connect the photoelectric device in parallel with the storage capacitance during the address period. During the driving period, the photoelectric device is effectively disconnected from the storage capacitance and plays no part in the operation of the pixel. In the address period, and with the data signal comprising a current signal, the photoelectric device serves to shunt current from the capacitance in dependence upon the display element""s light output until an equilibrium state is reached and the capacitance voltage, and thus the gate voltage of the drive TFT, stabilizes.
The photoelectric device preferably comprises a photodiode, although a photoresistor or phototransistor could alternatively be employed.